1. Field of the Invention
The present invention relates to a control device for temperature of a lamp, and a color direct thermal printer in use with the control device. More particularly, the present invention relates to a lamp temperature control device capable of maintaining intensity of radiation of the lamp while adjusting tube temperature of the lamp.
2. Description Related to the Related Art
In a color direct thermal printer, a color thermosensitive recording material is used, which is constituted of a support, a cyan coloring layer, a magenta coloring layer, and a yellow coloring layer, formed in the layered structure in the order listed, to print a full-color image directly according to the thermal printing, as disclosed, for example, in U.S. Pat. No. 4,734,704 (corresponding to JP-A 61-213169). In recording images by use of the color recording material, the uppermost layer at the obverse side is first subjected to recording. The thermal recording is performed in the order to the undermost layer at the reverse side. The coloring of a layer, of which the recording is finished, is fixed by application of ultraviolet radiation in a wavelength range appropriate for each layer, before the next underlying layer is subjected to the recording. This is for the purpose of invalidating the coloring ability and preventing the colored overlying layer from being colored again in an accidental manner.
To emanate ultraviolet radiation for fixation of the coloring, an ultraviolet lamp in a tubular shape is used. The lamp emanates radiation not only to the front but also to the rear. A reflector is disposed behind the lamp and apart therefrom, and is used for reflecting the rear radiation in order to utilize the performance of the lamp fully. The ultraviolet lamp, as disclosed in JP-Y 63-33321, has a radiating intensity which is changeable according to the temperature of a lamp tube. The tube temperature is different between a plurality of positions along the lamp. The electric current discharged in the lamp is conditioned by the lowest temperature among the tube positions. It follows that the overall radiating intensity of the lamp depends upon the lowest temperature. In general, the lamp has such characteristics that, if the tube temperature is lower, then the radiating intensity is lower; if the tube temperature rises, then the radiating intensity is raised; and when the tube temperature is extremely high, then the radiating intensity decreases. In view of stabilizing the fixation of the coloring, it is necessary to keep constant the ultraviolet radiating intensity, without being influenced by changes in the tube temperature of the lamp.
In the conventional thermal printer according to the color direct recording, there is disposed a sensor for detecting temperature, a lamp cooling fan and a heat discharging device, all near each lamp in order to control the radiating intensity of the lamp. Those devices as well as the above reflector are assembled via separately associated mounting members. The arrangement associated with the lamp for good fixation is involved in a problem of structural complication. A plurality of sensors must be associated individually with the lamps, so as to increase the cost of manufacture.
Another problem lies in that air is sent directly to the illuminating face of the lamp by the cooling fan. A limited portion receiving the cooling air is cooled to be the coolest portion among the portions on the lamp tube. Also, because mercury is enclosed in the tube, excessive cooling of the lamp deposits the mercury inside the tube at the coolest portion until the discharge of the ultraviolet radiation is hindered. Thereby, the radiating intensity will be lowered.